An electroluminescence element (hereinafter referred to as ‘EL element’), which is a capacitive light emitting element, can be electrically expressed as an equivalent circuit, as shown in FIG. 1. As can be understood from FIG. 1, an element can be substituted by a constitution of a capacity component C and a component E having a diode characteristic coupled in parallel to the capacity component. Therefore, an EL element can be considered to be a capacitive light emitting element. When a light emitting driving DC voltage is applied between the electrodes of the EL element, electric charge is accumulated in the capacity component C. When the voltage across the electrodes exceeds the barrier voltage or the light emission threshold voltage peculiar to the EL element, electric current starts flowing from the electrode (the anode side of the diode component E) to an organic functional layer forming a light emitting layer. As a result, the EL element emits light at light intensity proportional to the current.
The voltage V-current I-luminance L characteristic of the EL element is, as shown in FIG. 2, similar to the characteristic of a diode in that the current I is very small at a voltage lower than the light emission threshold voltage Vth and increases at a voltage higher than the light emission threshold voltage Vth. Further, the current I and the luminance L are nearly proportional to each other. The EL element shows light emitting luminance proportional to the current I which flows in accordance with a driving voltage V when the driving voltage applied to the EL element exceeds the light emission threshold voltage Vth, and shows no light emitting luminance when the driving voltage V applied to the EL element is equal to or lower than the light emission threshold voltage Vth.
When a voltage is applied to the EL element of which the function has deteriorated for repeated light emission, in a direction contrary to the forward direction, namely, a reverse bias voltage is applied, it is known that there is refresh action such that the function of the EL element is recovered.
As is also still known, in a light emitting circuit for making an EL element emit light, when a driving current is supplied to the EL element through a diode which is connected to the EL element in series, the EL element emits light, and then maintains the light emission by electric charge, which is accumulated in the capacitive component of the EL element in accordance with the driving current, for a while even after stopping the supply of the driving current. This phenomenon is can be effectively used in improving the average luminance of the EL element when a scanning time for every line is short in a display device for displaying through line scanning of a display panel with a plurality of EL elements arranged in a matrix shape, especially a display panel having a large number of lines.
In a light emitting circuit in which a diode is connected to an EL element in series, however, it is difficult to form a structure for applying a reserve bias voltage to the EL element to provide the refresh action.